1. Field of the Invention
The present invention generally relates to a trap circuit for removing a disturbance or interference wave, and more particularly to an image trap circuit for removing an image disturbance wave at an input stage such as an FM tuner or the like.
2. Description of the Related Art
In an FM tuner, a front end is constructed such that an image trap circuit for reducing a component of an image disturbance wave is inserted at the input stage, in order to prevent the deterioration of the receiving ability of the FM tuner due to the image disturbance wave corresponding to a desired wave having a predetermined frequency.
In this kind of image trap circuit, a parallel oscillator circuit is constructed such that a coil is connected between anodes of variable capacitance diodes, whose cathodes are connected to each other. A trimmer capacitor is also connected between the anodes of variable capacitance diodes in parallel to the coil. A tuning voltage is applied to the junction portion between the variable capacitance diodes, so that the capacitances of the variable capacitance diodes are varied by controlling this tuning voltage. The image frequency is set in correspondence with an oscillation frequency determined by the coil, the variable capacitance diodes and the trimmer capacitor. In operation, a received signal from an antenna is outputted to a main tuning circuit at the later stage thereof after the component of the image frequency is sufficiently reduced. Thus, it is possible to avoid the deterioration in the receiving ability of the circuit at the later stage due to the image disturbance.
Here, the function of the trimmer capacitor in this kind of image trap circuit is to restrain the variance or fluctuation of the image disturbance characteristic with respect to the tuning voltage, due to the variance in the respective elements. Namely, by finely adjusting the trimmer capacitor in the manufacturing process or the like, the relationship between the tuning voltage and the image frequency is maintained to be constant, so that it is possible to adjust the image disturbance characteristic in correspondence with the control width of the tuning voltage.
In this kind of image trap circuit, it is ideally preferable to give no influence onto a desired wave. However, actually in the frequency range of the desired wave, such a characteristic of having a peak at a certain portion is exhibited, and the passing loss is gradually increased in the range other than the peak. Since this peak is generated due to the characteristic of the above mentioned parallel oscillator circuit, when the trimmer capacitor is adjusted, not only the attenuation characteristic of the image disturbance frequency but also the peak generated in the frequency range of the desired wave is changed. By this, the level in the image trap circuit with respect to the desired wave is changed, so that the variation or irregularity in the receiving sensitivity of the FM tuner is brought about, which is a problem.
It is therefore an object of the present invention to provide an input trap circuit and an image trap circuit, which can keep the receiving sensitivity appropriate while restraining the level variation with respect to a desired frequency and can remove an unnecessary frequency component such as an image disturbance wave in stable, even if the frequency characteristic of the trap circuit at the input stage is finely adjusted by a variable capacitor element.
The above object of the present invention can be achieved by an input trap circuit for removing a predetermined frequency component included in an input signal and then outputting the input signal to a circuit at a later stage. The input trap circuit is provided with: a parallel oscillation circuit in which a pair of variable capacitance diodes are connected in series such that cathodes thereof are connected to each other at a junction portion and in which an inductance is connected in parallel to the variable capacitance diodes between anodes of the variable capacitance diodes at both ends thereof; a voltage applying device for applying a voltage to the junction portion so as to vary capacitance values of the variable capacitance diodes on the basis of reverse voltage characteristics of the variable capacitance diodes respectively; and a variable capacitor element connected between the junction portion and a ground.
According to the input trap circuit of the present invention, when the input signal is inputted to the input trap circuit, it passes through the parallel oscillation circuit having the pair of variable capacitance diodes and the inductance such as a coil etc., and is outputted to the circuit at the later stage in such a condition that the frequency component corresponding to the oscillation frequency is attenuated. At this time, with respect to the junction portion where the cathodes of the pair of variable capacitance diodes are connected to each other, the predetermined voltage is applied by the voltage applying device, so that the capacitance values are changed on the basis of the reverse voltage characteristics of the variable capacitance diodes. Therefore, it is possible to set a predetermined unnecessary frequency to the oscillation frequency, and thereby remove the unnecessary frequency component easily out of the input signal. On the other hand, by the variable capacitance element connected between the junction portion and the ground, the characteristic for the frequency to be removed can be adjusted. At the time of adjusting it, the characteristic for the frequency other than the frequency to be removed can be kept in stable. Thus, without causing a level change of the input signal, it is possible to adjust the frequency characteristic of the input trap circuit appropriate for removing the above mentioned unnecessary frequency component.
In this manner, according to the present invention, it is possible to provide the input trap circuit, which can easily remove the unnecessary frequency component by controlling the frequency characteristic of the oscillator circuit, and also can adjust the characteristic of the frequency to be removed appropriate by the variable capacitor element without causing the level change in the passing component.
The above object of the present invention can be also achieved by an image trap circuit, which is inserted in an input stage of an FM tuner circuit, for removing an image frequency component included in a received signal and then outputting the received signal to a circuit at a later stage. The image trap circuit is provided with: a parallel oscillation circuit in which a pair of variable capacitance diodes are connected in series such that cathodes thereof are connected to each other at a junction portion and in which an inductance is connected in parallel to the variable capacitance diodes between anodes of the variable capacitance diodes at both ends thereof; a voltage applying device for applying a voltage to the junction portion so as to vary capacitance values of the variable capacitance diodes on the basis of reverse voltage characteristics of the variable capacitance diodes respectively; and a variable capacitor element connected between the junction portion and a ground, wherein a voltage variable range of the voltage applying device is set in correspondence with a frequency band of the image frequency component.
According to the image trap circuit of the present invention, when the received signal is inputted to the FM tuner circuit, it passes through the parallel oscillation circuit having the pair of variable capacitance diodes and the inductance such as a coil etc., and is outputted to the circuit at the later stage in such a condition that the frequency component corresponding to the oscillation frequency is attenuated. At this time, with respect to the junction portion where the cathodes of the pair of variable capacitance diodes are connected to each other, the predetermined voltage is applied by the voltage applying device, so that the capacitance values are changed on the basis of the reverse voltage characteristics of the variable capacitance diodes. Therefore, by setting an image frequency corresponding to the received signal to the oscillation frequency, it is possible to remove the image frequency component easily out of the received signal and thereby improve the image disturbance characteristic easily. On the other hand, by the variable capacitance element connected between the junction portion and the ground, the characteristic for the image frequency can be adjusted. At the time of adjusting it, the characteristic of the frequency of the desired wave can be kept in stable. Thus, without causing a level change of the received signal, it is possible to adjust the image disturbance characteristic of the image trap circuit appropriate while maintaining the receiving sensitivity high.
In this manner, according to the present invention, it is possible to provide the image trap circuit, which can easily remove the image frequency component corresponding to the FM received signal by controlling the frequency characteristic of the oscillator circuit, and also can adjust the removing characteristic for the image frequency appropriate by the variable capacitor element while maintaining the receiving ability high for the FM received signal.
In one aspect of the image trap circuit of the present invention, a mixer circuit included in the FM tuner circuit generates an IF (Intermediate Frequency) signal by use of a local oscillation frequency, which is higher than a frequency of the received signal.
According to this aspect, in the FM tuner circuit, the frequency, which is higher than the of the received signal by the frequency of the IF signal, is set as the local oscillator frequency, and is mixed by the mixer circuit so that the IF signal is generated. Thus, the image disturbance wave is generated at the frequency, which is higher than the local oscillator frequency by the frequency of the IF signal. As a result, the frequency, which is higher than the frequency of the received signal by two times of the frequency of the IF signal, becomes the image frequency. Under such an FM receiving condition in general, it is possible to maintain the receiving sensitivity high and also adjust the image disturbance characteristic appropriate.
In this manner, according to the present invention, it is possible to provide the image trap circuit, which can adjust the removing characteristic for the image frequency appropriate by the variable capacitor element while maintaining the receiving ability high for the FM received signal, even in case that the frequency of the FM received signal is lower and the image frequency is higher than the local oscillation frequency, which is the general frequency relationship in the FM tuner.
The nature, utility, and further features of this invention will be more clearly apparent from the following detailed description with respect to preferred embodiments of the invention when read in conjunction with the accompanying drawings briefly described below.